All electronic circuits require some source of power from, which to operate. Therefore, a power supply is an essential element of every electronic system.
In recent years, Switch Mode Power Supplies (SMPS) have become a staple of the electronics industry. An example of this type of power supply is disclosed and claimed in U.S. Pat. No. 4,282,567 issued to Voigt. A feature of switch mode power supplies is that they can operate at high frequencies and thus can operate more efficiently using smaller and lighter-weight components than conventional (line frequency) power supplies. Another feature is that a feedback circuit from an output to a switching regulator is used, thereby eliminating the need for a separate inefficient series regulator on the voltage output used as the feedback reference.
It is frequently desired that power supplies be constructed so as to produce multiple simultaneous voltage 33 outputs such as +5VDC, -12VDC, and +12VDC. With conventional power supplies this was accomplished using multitap or multiwinding transformers which produced a variety of different voltage outputs depending upon the relative location of the "taps" or number of turns on the transformer secondary winding. However, this method is impractical when applied to higher-frequency switch mode power supplies, since the frequency dictates use of very low inductance transformer windings (frequently comprising only one turn on the secondary winding).
Available output voltages from a transformer are a function of the ratio of the number of turns in the secondary winding to the number of turns in the primary, according to the general formula: ##EQU1## Vs=voltage output from the transformer secondary winding Vp=voltage input to the transformer primary winding
Ns=number of turns on the transformer secondary winding PA0 Np=number of turns on the transformer primary winding
Therefore, if Ns is restricted to one turn, obviously there can be but a single Vs for any given combination of Vp and Np.
In commercial switch mode power supplies currently being manufactured, the solution to this problem has been to design the power supply to produce an AC output at the transformer secondary (or at the switching regulator output, if no output transformer is used) sufficiently high such that it can be rectified and filtered to produce the highest DC voltage desired at the output. Lower voltages are then produced by means of additional single component regulators or regulator circuits for each additional output desired. Alternatively, especially where the desired voltages are very dissimilar, a transformer is configured so as to produce one selected voltage relatively precisely, with that voltage being the feedback voltage to the switching regulator circuit of the power supply. The remainder of the desired voltages are produced at the output of the transformer as high approximations of the desired voltages as best as can be obtained by a whole number of winding turns. For instance, a transformer with a 60-volts potential across the primary winding and desired outputs of 5 volts and 12 volts could use 12 primary windings and one secondary winding to accurately produce the 5-volt output with that output being fed back to the switching regulator circuit and with no additional regulation being required on that voltage output. But, since only complete turns are practical on the secondary, the 12-volt output has to be produced by a 3-turn secondary with a 15-volt output, which 15 volts is then regulated by a supplementary series regulator down to the desired 12 volts. But these approaches have several disadvantages:
1) They require additional relatively expensive components.
2) There is a significant sacrifice of efficiency at each independently regulated output, the output efficiency of each such regulator being inversely proportional to the difference between the regulator input voltage and the regulated output voltage.
3) The available power at each output is limited by the power handling capability of the regulator itself.
4) The reliability of the unit is considerably reduced, since the regulators are more prone to failure than many of the other power supply components, and since any increase of active component count will result in a statistical reduction of reliability.
5) Production of negative voltages (as referenced to the "main" power supply DC output voltage) may require additional circuitry.
6) The combined current draw from the transformer might require a larger or more expensive transformer core in order to prevent saturation.
7) The power dissipation in series or shunt regulators produces a considerable amount of heat, thus requiring the use of heat sinks, air circulation space, and wider component spacings, all of which increase the size of the unit.
All of the prior art multiple output switch mode power supplies within the inventor's knowledge have utilized supplementary independent series or shunt regulators for each output required in addition to the control or "feedback" voltage, unless it just happened that auxiliary outputs might be derived from existing voltages without the use of such supplementary regulators. No prior art switch mode power supply to the inventor's knowledge has produced an output additional to its main feedback voltage, except perhaps another voltage coincidentally related to that feedback voltage by a ratio of N1/N2 where both N1 and N2 are integers, without the use of separate regulator circuits on each such additional output.